It is known that in the case of picture tubes which are driven by a high video signal amplitude, such as for example, projectors, the video amplifiers for driving the picture tubes are designed for relatively high supply voltages and output levels. A limit is created by the maximum voltage and temperature loading of the transistors. However, the video signals with the high frequency requirements on the output stage occur only in the rarest cases, such as for example in the case of frequency response measurements or special computer graphics. Therefore, designing the output stage for these relatively high loads tends to be detrimental in normal operation, since the power consumption and the generation of heat increase. The cause of this problem lies in the temperature dependence of the transistor on the frequency, since the current, and consequently the temperature, increase linearly at high frequencies.
Furthermore, the operating point of the video amplifier shifts at high levels of high-frequency signal output, and then low-frequency signal components are distorted when they are transmitted. In practice this has the effect that the return lines of the sweep circuits and various picture brightness defects with superimposed bar patterns or shades of grey are evident. An increasing of the quiescent current for the operating point of the video amplifier would indeed solve the problem, but would have the known power and temperature problems as a consequence.
Therefor, there is a need to for a video amplifier stage which provides a relatively distortion free display of video signals with little outlay on the components.